A cornerstone in the diagnosis of heart abnormalities is the electrocardiograph. With a typical electrocardiograph, conductive leads connect electrodes placed on the patient to a stationary measuring machine while the patient rests on a cot or the like.
For episodic abnormalities, the electrocardiogram ("ECG") recorded by the electrocardiograph desirably covers a period of time preceding the episode ("pre-symptom data") and after the episode ("post-symptom data"). Yet immobilizing the patient during the acquisition of such pre-symptom and post-symptom data is impractical because of the general lack of predictability as to the time of occurrence of the episode. In such situations, therefore, it is known to employ a portable ECG recorder that allows the patient to be ambulatory during the measurement period. Such portable ECG equipment employs a magnetic tape to capture data during a period as long as 24 hours.
With advances in computer electronics, the so-called "memory loop recorder" has been developed. The memory loop recorder samples and digitizes the ECG signal which may then be stored as numeric values in a computer memory. Such computer memories typically have a capacity less than that obtainable by use of magnetic tape storage, however, the computer memory may be readily operated to store the ECG signal in a loop fashion over an indefinite period of time. For example, at any given time the memory loop recorder might store the last twenty minutes of ECG data. Newly arriving data continues to replace the oldest data in the memory.
The patient is provided with a control that is to be activated when an episodic abnormality is detected. At this time the recording continues for a predetermined time and then stops so that the memory holds both pre- and post-symptom data.
As any episodic cardiac abnormality may prove to be life-threatening, it is desirable to have the recorded ECG data reviewed by an electrocardiographer as soon as possible. In this regard, it is known to employ telephone line for the transmission of recorded ECG data to a remote display device where it can be reviewed and analyzed. In such systems, the stored ECG data is played back over the telephone line from beginning to end so that the electrocardiographer can view the progression of the abnormal episode.
The maximum loop length for each episode may be up to twenty minutes long and, therefore, the electrocardiographer may have to wait a significant period of time to see the data immediately preceding the abnormality. This delay may be critical in some cases and may delay the dispatch of emergency personnel. Any reduction in the length of the loop risks the loss of critical diagnostic data and requires the patient to rapidly recognize and respond to the onset of the abnormality.